1. Field of the Invention
The present invention generally relates to electro-optical readers and, more particularly, to a compact scan engine having a dual chip architecture for use in such readers, the dual chip architecture being readily configurable and operative for controlling reader operation.
2. Description of the Related Art
Various electro-optical readers have been developed heretofore for reading indicia such as bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers function by electro-optically transforming the spatial pattern represented by the graphic indicia into a time-varying electrical signal, which is in turn decoded into data which represent the information or characters encoded in the indicia that are related to the article or some characteristic thereof.
Such data is typically represented in digital form and utilized as an input to a data processing system for applications in point-of-sale processing, inventory control, distribution, transportation and logistics, and the like. Readers of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; 4,896,026; 5,015,833; 5,262,627; 5,504,316; 5,625,483; and 6,123,265, all of which have been assigned to the same assignee as the instant application and each of which is hereby incorporated by reference herein.
A typical reader includes, inter alia, a hand-held, portable laser scanning device supported by a user. The user aims the device and, more particularly, a light beam, at a targeted symbol to be read. The light source in the reader is typically a semiconductor laser energized by a laser drive circuit. The use of semiconductor devices as the light source is especially desirable because of their small size, low cost and low voltage requirements. The laser beam is optically modified, typically by an optical assembly, to form a beam spot of a certain size at the target distance. It is often preferred that the cross-section of the beam spot measured in the scanning direction at the target distance be approximately the same as the minimum width in the scanning direction between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
In the known readers, a laser light beam is directed by a lens or other optical components along the light path toward a target that includes a bar code symbol on the surface. A moving-beam reader operates by repetitively scanning the light beam in a line or series of lines across the symbol by means of motion of a scan element or scanning component, such as the light source itself or a mirror disposed in the path of the light beam, driven by a motor drive circuit. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the pattern of the symbol, or scan the field of view of the reader, or do both.
The reader also includes a sensor or photodetector which detects light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol, detected, and converted into an electrical signal.
In retroreflective light collection, a single optical component, e.g., a reciprocally oscillatory mirror, such as described in U.S. Pat. No. 4,816,661 or U.S. Pat. No. 4,409,470, both herein incorporated by reference, scans the beam across a target surface and directs the collected light to a detector. In non-retroreflective light collection, the reflected laser light is not collected by the same optical component used for scanning. Instead, the detector is independent of the scanning beam, and is typically constructed to have a large field of view so that the reflected laser light traces across the field of view of the detector.
Electronic receiver circuitry receives the electrical signal and a digitizer processes the signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector may be converted by the digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Alternatively, the analog electrical signal may be processed directly by a software decoder. See, for example, U.S. Pat. No. 5,504,318.
The decoding process usually works in the following way. The analog signal from the sensor or photodetector is amplified, filtered and processed by a receiver circuit which includes an automatic gain control circuit. The pulse width modulated digitized signal is applied to a software algorithm, which attempts to decode the signal, in a microprocessor. If the start and stop characters and the characters between them in the scan were decoded successfully and completely, the decoding process terminates and an indicator of a successful read (such as a green light and/or audible beep) is provided to the user. Otherwise, the decoder implemented as software in the microprocessor receives the next scan, and performs another decode according to a symbology specification into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
The binary data is communicated to a host computer by an interface cable or wireless communication link. The interface cable may be a “smart cable” such as that described in U.S. Pat. No. 5,664,229 and U.S. Pat. No. 5,675,139, the contents of which are hereby incorporated by reference herein.
The bar code symbols are formed from bars or elements typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or “symbology” used. The relative size of the bars and spaces is determined by the type of coding used as is the actual size of the bars and spaces. The number of characters (represented by the bar code symbol) per unit length is referred to as the density of the symbol. To encode the desired sequence of the characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies, a unique “start” and “stop” character is used to indicate when the bar code begins and ends. A number of different bar code symbologies is in widespread use including UPC/EAN, Code 39, Code 128, Codeabar, and Interleaved 2 of 5.
In order to increase the amount of data that can be represented or stored on a given amount of surface area, several more compact bar code symbologies have been developed. One of these code standards, Code 49, exemplifies a “two-dimensional” symbol by reducing the vertical height of a one-dimensional symbol, and then stacking distinct rows of such one-dimensional symbols, so that information is encoded both vertically as well as horizontally. That is, in Code 49, there are several rows of bar and space patterns, instead of only one row as in a “one-dimensional” symbol. The structure of Code 49 is described in U.S. Pat. No. 4,794,239. Another two-dimensional symbology, known as “PDF417”, is described in U.S. Pat. No. 5,304,786.
For safety reasons, there are regulations concerning the maximum level of laser beam output power that can be emitted from an electro-optical reader, and this maximum level depends on whether or not the reader is operative to shut off the laser upon detection of a failure of the drive that oscillates the scan element. For example, a Class II laser safety device is limited to a maximum laser output power of 1 mw if there is no laser shut-off feature. If there is a laser shut-off feature, then the device is permitted to output a higher power in the emitted stationary laser beam. A higher laser beam output power is desirable for an increased working range, ambient light immunity, greater scan line visibility and, in general, better overall reader performance.
It is known in the art to use a hardware safety circuit to detect drive failure. The drive includes a motor and a feedback winding that generates a feedback signal having a voltage or amplitude. The hardware safety circuit monitors the amplitude of the feedback signal and turns the laser off if the amplitude falls below a predetermined threshold, thereby indicating that the drive is malfunctioning.
Although generally satisfactory for their intended purposes, the above-described laser drive circuit, motor drive circuit, receiver circuit, digitizer circuit and safety circuit are realized by discrete electrical components that occupy non-negligible space in, and add extra weight to, the reader. These circuits are implemented on one or more printed circuit boards which, together with the other above-described electrical and optical components of the reader, constitute an assembly or module, typically referred to as a “scan engine”. It is desirable for the scan engine to be used interchangeably in a variety of different operating modalities and readers of different form factors. Yet, it is difficult to configure the discrete electrical components, which typically are configured manually, especially in the field. The known scan engines are simply too bulky and heavy for those applications where an ultra small form factor is required. Also, the mounting of discrete electrical components increases the assembly cost.